Effects of thermal gradients in sapwood on stem psychrometry

Water potential measurements are considered the gold standard for determining plant water status and drought stress in nature and horticulture. Plant water potentials are routinely measured destructively using pressure chambers, but that method is labor-intensive and the resulting data are discrete in time. Stem psychrometers, on the other hand, allow monitoring plant water potentials continuously. They are notoriously temperature-sensitive, with a gradient of 1°C between plant and instrument temperature causing a deviation in water potential of up to 8 MPa. To quantify potential effects of thermal gradients in sapwood on stem psychrometry we chose two deeply-rooted chaparral shrubs in southern California as our study systems, Malosma laurina and Heteromeles arbutifolia. Depending on the time of day, sap ascending from deep roots can potentially be much colder or warmer than the air surrounding a stem, thereby creating thermal gradients at the sapwood surface. These gradients were measured with small thermistors placed at different distances from the exposed sapwood surface, while sapwood water potentials were measured with stem psychrometers. Substantial thermal gradients were detected in both species, in some cases exceeding 0.1°C mm-1, with sapwood temperature fluctuating diurnally from being warmer or colder than the outside air. Effects of these gradients on psychrometric water potential readings were complex and varied with the time of day and the direction of the gradient, causing false readings deviating by more than 2 MPa from correct values determined by concurrent pressure chamber measurements. Attempts to simultaneously measure and correct for these gradients were unsuccessful, but placement of stem psychrometers far away from the stem base largely alleviated the problem by allowing sapwood temperatures to equilibrate with air temperatures. We therefore recommend installing stem psychrometers as far away from the stem base as possible and always compare initial readings to pressure chamber measurements for validation.